1. Field of the Invention
The present invention relates to a structure for connecting a compressor wheel and a shaft.
2. Description of the Related Art
As means for compressing air to increase the amount of intake air to an engine, a compressor of a turbo machine which rotates a turbine wheel and a shaft by utilizing energy of exhaust gas and drives a centrifugal type compressor wheel connected with the shaft is known as a turbo charger.
FIG. 11 is a sectional side view of a turbo charger 111 according to the related prior art. The turbo charger 111 includes an exhaust-side unit 112 for gaining rotational energy from the exhaust gas of an engine and an intake-side unit 113 for compressing air by the rotational energy and supplying the compressed air to the engine.
A turbine wheel 114 receives energy from the exhaust gas flowing thereto from an exhaust inflow passage 119 and rotates by the energy. A centrifugal type compressor wheel 116 for compressing air via a shaft 123 is fitted to the shaft 123 on a side opposite to the turbine wheel 114, i.e., the tip of the shaft 123.
A fitting hole 125 penetrates through a center of the compressor wheel 116. The shaft 123 is fitted into the fitting hole 125 by slight clearance fit or close fit. The compressor wheel 116 is fixed to the shaft 123 by fastening a fitting nut 126 to a male screw 140 formed at the tip of the shaft 123.
FIG. 12 is a sectional side view of the compressor wheel 116 according to the related art. A main body 129 of the compressor wheel 116 includes an inlet-side disk portion 129A and a back-side disk portion 129B. A plurality of vanes 118 are arranged outside the main body 129, and the fitting hole 125 penetrates through the center of the main body 129.
The compressor wheel 116 is produced from a casting such as an aluminum alloy or other material so as to be light-weight. Since the rotating speed of the compressor wheel 116 reaches values as high as tens of thousands rpm, extremely high tensile stress is applied on the compressor wheel 116 in its radial direction due to centrifugal force generated by the high rotating speed and thus the compressor wheel 116 may be broken in some cases.
It is known that the breakage of this type is likely to develop particularly in the inner wall of the fitting hole 125 starting therefrom. More specifically, it has been clarified that the breakage of the inner wall of the fitting hole 125 formed on the compressor wheel 116 occurs particularly in the vicinity of a maximum outer diameter 130 where the outer diameter of the compressor wheel 116 reaches a maximum in an axial direction of a rotational axis of the compressor wheel 116.
In order to solve this problem, a technology described in Patent Reference No. JP-T-5-504178 (the term “JP-T” as used herein means a published Japanese translation of a PCT patent application. pp. 3 to 5, FIGS. 1 and 2), for example, is utilized.
FIG. 13 is a cross-sectional view of a compressor wheel 216 according to the patent reference. A fitting hole penetrating through the compressor wheel 216 is not provided but a fitting opening 242 having a female screw is formed at a lower region of the compressor wheel 216. A male screw is provided at a tip 254 of a shaft 223. The shaft 223 and the compressor wheel 216 are coupled with each other by screwing the tip 254 into the fitting opening 242.
However, since the fitting opening is also provided in the vicinity of the maximum outer diameter where the outer diameter of the compressor wheel reaches a maximum in the axial direction of the rotational axis of the compressor wheel in the related art shown in the patent reference, there is a possibility of breakage starting from a region around the maximum outer diameter when the rotating speed is increased.
Particularly when an engine equipped with the turbo charger using the compressor wheel is employed in working machines such as construction machines, a high load condition such as a loading operation (a high rotating speed of the engine) and an almost no load condition (a low rotating speed of the engine) are alternately repeated at short intervals.
As a result, the stress amplitude applied to the compressor wheel increases and the breakage is more likely to occur.
Recently, a technology called “EGR” (Exhaust Gas recirculation) has been executed as measures for the reduction of nitrogen oxides (NOx) contained in exhaust gas of Diesel engines. In this method, a part of exhaust gas discharged from an engine is returned to an intake system of the engine for re-circulation.
For accomplishing EGR, it is necessary to achieve a higher pressure ratio of the turbo charger so as to secure combustion air from a capacity of fresh air within a cylinder which capacity is reduced by the amount of the re-circulated exhaust gas, and thus the rotating speed at which the compressor wheel is rotated needs to be increased. However, the related art is not sufficient to overcome the above problem and it is thus desired to develop a compressor wheel having higher durability.